Diameters of metal bodies, in particular of wires, are determined for various purposes.
Determining the diameter of a wire is important, for example, in the production of incandescent lamps. The wires used for production of incandescent lamps must have an approximately constant diameter, otherwise there is the risk that the wires are unsuitable for further processing at the points at which the diameter is too large or too small. The wires which are used to produce incandescent lamps have a diameter in the range of typically 8-500 .mu.m. It is impossible in practice to measure the diameter of these wires in a direct mechanical fashion, for example by means of a micrometer screw. One method of the prior art consists in weighing wire segments of defined length, for example 200 mm, in order to derive the average diameter of the wire segment from the weight (weighing method).
Since the quality of an incandescent lamp depends on its service life and the latter depends inter alia, on the uniformity of the tungsten wire used for the incandescent filament, when producing incandescent lamps it is of great importance already to establish in an early stage of the production, specifically when providing and cleaning the wire, which wire sections are unsuitable for further processing. Wire sections whose diameters are above or below a specific desired value are unsuitable.
In the prior art, an oscillator which generates a radio-frequency (RF) signal in the megahertz band has its frequency detuned by the means of a measuring capacitor (capacitor 11; in FIGS. 1a, 1b) whose capacitance varies with the diameter of the wire guided through. An output signal is formed from this detuning, and a wire diameter deviation is determined (RF micrometer) with respect to an average diameter which can be determined by means of the abovementioned weighing method. If the diameter at the start of the wire is determined using the weighing method and combined with the measured relative values, the result is a data record with absolute wire diameters.
It is thus possible to use the circuit arrangement to determine whether a specific wire segment is suitable for the production of a specific type of incandescent lamp.
The RF micrometer according to the prior art has the disadvantage that the output signal which is to represent the diameter deviation of the wire is unstable, the instability being the result of fluctuating environmental parameters (temperature, atmospheric humidity, etc.) which influence the dielectric constant of the measuring capacitor and thus the capacitance of the latter. These problems exist, in particular, at the fabrication shops for the tungsten wires, where the room temperature is influenced by a multiplicity of parameters (thermal emission of the machines, etc.) which can scarcely be mastered in practice. This severe disadvantage in the prior art is remedied by carrying out the measurement in a conditioning room screened from the environment. Aside from the fact that such a measure is very expensive in terms of energy and cost, it is impossible for this method to be used for automatically testing directly at the wire fabrication shops.